A) Field of the Invention
The present invention relates to an optical device and its manufacture method, and more particularly to an optical device having diffraction gratings coupling guided wave propagating in an optical waveguide.
B) Description of the Related Art
With an explosive increase in demand for Internet, ultra high speed and large capacity have been studied vigorously in the fields of optical communications and transmissions. Inexpensive semiconductor laser devices capable of direct modulation at a frequency not lower than 10 Gb/s without cooling have been desired, particularly for Ethernet (registered trademark) having a giga bit transmission band. A semiconductor laser device meeting these needs includes a distributed feedback (DFB) type laser device.
In order to manufacture a DFB laser device with low cost, a ridge type laser device is promising which can be manufactured by crystal growth in a single process, i.e., which does not require another process for crystal growth after an etching process. It is advantageous from the viewpoint of manufacture cost that diffraction gratings for distributed feedback of a ridge type laser device are formed not in a bosom of a crystal but on both sides of the ridge.
FIG. 16 is a perspective view of a conventional ridge type DFB laser device. On a semiconductor substrate 500, an active layer 501 and a cladding layer 502 are sequentially stacked. A ridge 503 extending in one direction is formed on the clad layer 502. Diffraction gratings 504 are formed on the sidewalls of the ridge 503. Part of the active region 501 down below the ridge 503 serves as an optical waveguide.
FIG. 17 shows another example of a conventional ridge type DFB laser device. In the ridge type DFB laser device shown in FIG. 16, the diffraction gratings 504 are formed on the sidewalls of the ridge 503, and in the example shown in FIG. 17, diffraction gratings 504A are formed on a flat surface on both sides of the ridge 503, substituting for the diffraction gratings 504. The other structures are the same as those of the laser device shown in FIG. 16.
FIG. 18 shows a positional relation between guided wave propagating in an optical waveguide and diffraction gratings. The diffraction gratings 504 or 504A are disposed on both sides of the ridge 503. A light intensity distribution of a guided wave in the fundamental transverse mode has a maximum intensity at the center of the ridge 503 in a width direction, and the light intensity lowers as the distance from the center, as indicated by a solid line 510. A light intensity distribution in the first higher order transverse mode (hereinafter abbreviated to “second-order transverse mode”) has a minimum intensity at the center of the ridge 503 in a width direction, the light intensity increases as the distance from the center, and the light intensity distribution has maximum intensities on both sides of the center, as indicated by a solid line 511. In the region outside the maximum intensity positions, a light intensity lowers monotonously as the distance from the center of the ridge 503. No diffraction grating is disposed near the center of the ridge 503 but the diffraction gratings are disposed on both sides of the ridge 503. A light intensity of the second-order transverse mode is therefore stronger than that of the fundamental transverse mode, in the regions where the diffraction gratings are disposed. Therefore, a coupling coefficient between the second-order transverse mode and the diffraction gratings is about 1.5 to 2 times as large as that between the fundamental transverse mode and the diffraction gratings. Oscillation of the second-order transverse mode is therefore likely to occur.
In order to lower the coupling coefficient between the second-order transverse mode and the diffraction gratings, it is effective to make the ridge 503 narrower to set the diffraction gratings near at the center of the ridge 503. However, as the ridge 503 is made narrow, an electric resistance of the laser device increases. Narrowing the ridge 503 may cause an increase in consumption power and a reduction in optical output due to heat generation when large current is injected.
JP-A-2003-152273 discloses a semiconductor laser device capable of suppressing high-order transverse modes.
FIG. 19 is a plan cross sectional view of a ridge portion of a semiconductor laser device disclosed in JP-A-2003-152273. Diffraction gratings 521 are formed on the sidewalls of a ridge 520. A light absorption layer 522 made of InGaAs having absorbability of oscillation light is formed on concave/convex outer surfaces of the diffraction gratings 522. Since the light absorption layer 522 absorbs the high-order transverse modes more than the fundamental transverse mode, oscillation of the high-order transverse modes can be suppressed.